1. Field of the Invention
The invention relates to an image forming apparatus, specially relates to an image forming apparatus which expresses gray scale data through repeating a plurality of lamp lighting patterns corresponding to different positions.
2. Related Background Art
Conventionally, in an image forming apparatus such as printer with electrophotography and the like, in order to lowly inhibit cost and increase gray scale, through repeating on/off of lighting lamp of plural sub-dots corresponding to different assistant scanning positions, lamp lighting energy corresponding to input gray scale data is expressed. Such technical skill is disclosed in patent document 1 (refer to patent document 1).
Such image forming apparatus comprises a gray scale value inputting register to input gray scale data of n-bits and a dot forming section which forms sub-dots on plural of sub-lines to corresponding to the inputted gray scale value of n-bits received from the gray scale value inputting register. With respect to each sub-line, the dot forming section sets weight factor to corresponding lamp lighting time. That is, with respect to the same sub-line, a fixed weight factor is set. Regarding such example, it will be explained in detail by using drawings.
FIG. 12 is an explanation diagram of inputted gray scale value.
The FIG. 12, as an example, shows gray scale value of 4-bits received by the above-stated conventional image forming apparatus via the gray scale value inputting register. In the FIG. 12, the Y-axis direction indicates an assistant scanning direction, on the most left queue, respective line numbers are shown; and the X-axis direction indicates an main scanning direction, on the highest row, respective pixel numbers on each line are shown; further, on each column, a gray scale value of pixel specified by the corresponding line number and the corresponding pixel number is stated.
The FIG. 12, as an example, shows a case that gray scale values are orderly inputted to the gray scale value inputting register. In the case, as shown by the FIG. 12, at timing of line 1, the gray scale values (1, 1, 1, 1, 1, 1, 1, 1, 1, 1) is inputted; at timing of line 2, the gray scale values (1, 1, 1, 1, 1, 1, 1, 1, 1, 1) is inputted; at timing of line 3, the gray scale values (1, 1, 1, 1, 1, 1, 1, 1, 1, 1) is inputted; at timing of line 4, the gray scale values (13, 13, 13, 13, 13, 13, 13, 13, 13, 13) is inputted; at timing of line 5, the gray scale values (13, 13, 13, 13, 13, 13, 13, 13, 13, 13) is inputted; at timing of line 6, the gray scale values (13, 13, 13, 13, 13, 13, 13, 13, 13, 13) is inputted; at timing of line 7, the gray scale values (1, 1, 1, 1, 1, 1, 1, 1, 1, 1) is inputted; at timing of line 8, the gray scale values (1, 1, 1, 1, 1, 1, 1, 1, 1, 1) is inputted; and at timing of line 9, the gray scale values (1, 1, 1, 1, 1, 1, 1, 1, 1, 1) is inputted.
FIG. 13 is an explanation diagram of a relation between a gray scale value and a standardization exposure energy of each pixel.
The FIG. 13 shows a relation between a gray scale value k (X-axis) and a standardization exposure energy es(k) (Y-axis) with respect to an actual LED printer. Here, the standardization exposure energy es(k) is a 8-bits value, it is obtained through that the image forming apparatus having received the inputted gray scale data standardizes an exposure energy e(k) necessary for realizing the gray scale value k by using a maximum exposure energy E(w) corresponding to one LED pixel, that is, es(k)=(e(k)/E(w))256. According to the relation shown by the FIG. 13, with respect to the gray scale value 1 of the above-stated lines 1˜3 and lines 7˜9, es(1)=63, and with respect to the gray scale value 13 of the above-stated lines 4-6, es(13)=192, they can be obtained. When using 8-bits of s[0]˜s[7] to express these values, the bit data of es(1) becomes [11111100], and the bit data of es(13) becomes [00000011].
FIG. 14 is an explanation diagram of a conventional pixel forming method.
In the FIG. 14, the Y-axis direction indicates an assistant scanning direction, on the most left queue, respective line numbers are shown; and the X-axis direction indicates an main scanning direction, on the highest row, respective pixel numbers on each line are shown. Here, in the assistant scanning direction and the main scanning direction, when they are all set that the pixel is formed by a pitch of 1/600 inch, 8 sub-lines (0, 1, 2, 3, 4, 5, 6, 7) will be formed in the assistant scanning direction per 1/4800 inch. The standardization exposure energy es(k) of each pixel is divided into bit data of 8-bits, and they, as sub-dots, are respectively indicated on the 8 sub-lines (0, 1, 2, 3, 4, 5, 6, 7).
With respect to the size (diameter) of sub-dot, in the sub-line 0, a weight factor of 1 is set; in the sub-line 1, a weight factor of 2 is set; in the sub-line 2, a weight factor of 4 is set; in the sub-line 3, a weight factor of 8 is set; in the sub-line 4, a weight factor of 16 is set; in the sub-line 5, a weight factor of 32 is set; in the sub-line 6, a weight factor of 64 is set; and in the sub-line 7, a weight factor of 128 is set. Thus, from the lines 1˜3 and 7˜9, the above-stated es(1) is indicated, and from the lines 4˜6, the above-stated es(13) is indicated. Therefore, when making the size (diameter) of sub-dot correspond to strobe time, on each same sub-line, the strobe time is identically set, and it becomes unnecessary to set strobe time per pixel on the corresponding line. As a result, it also becomes unnecessary to set memory for memorizing strobe time per pixel, then it is possible to lowly inhibit cost and to increase the gray scale of each pixel.
Patent document 1: Japan patent publication 2005-22410.
However, as shown by the FIG. 14, on the one hand, in the border neighborhood of line 3 and line 4, because adjoining sub-dots often happen overlap, so an average density in the main scanning direction becomes high. On the other hand, in the border neighborhood of line 6 and line 7, because adjoining sub-dots do not happen overlap, so an average density in the main scanning direction becomes low. As a result, when macroscopically observing the FIG. 14, as shown on the right side in the FIG. 14, on the border neighborhood of line 3 and line 4, a thin black stripe is happening; and on the border neighborhood of line 6 and line 7, a thin white stripe is happening. That is, in the assistant scanning direction, with respect to such image that rapidly changes from a state close to white level toward a state close to black level, on the border portion, a thin black stripe easily happens; on the contrary, with respect to such image that rapidly changes from a state close to black level toward a state close to white level, on the border portion, a thin white stripe easily happens. Thereby, such problem to be solved is left.